In technology for estimating axis error in a position sensor-less vector control method, as described in Japanese Patent Laid-open No. 2001-251889, voltage command values, detected current values, and a speed command value, which are outputs in vector control, are used to estimate axis error by calculation, and the calculated value is then used to estimate a frequency by calculation. The description in the control method also stipulates that command values for axis error are basically set to 0.